This invention relates to mechanical sorting systems for sorting objects according to the degree to which they possess a certain characteristic. The invention has particular, but not exclusive, application to ore sorting equipment for sorting ore rocks.
There are various types of ore sorting equipment in which the ore rocks to be sorted are moved in a stream past one or more detectors responsive to a selected characteristic. A decision is made on the suitability of each object by examining the detector output and the object is then accepted or rejected. The detectors may be photometric devices to measure surface characteristic of the rocks or they may measure other characteristics such as radioactivity, electrical resistivity or magnetic permeability.
In conventional ore sorting equipment the rocks are deposited on a fast moving horizontal conveyor belt which moves them in a stream past the detecting or scanning equipment. The rocks are projected from the end of the conveyor in a free flight trajectory and they are either accepted or rejected by being deflected from that trajectory either by a movable deflector plate or by air blasts delivered by a series of blast nozzles.
In some systems the rocks are scanned while in free flight to measure the characteristic to be used as the basis for sorting. In other systems, the rocks pass appropriate detectors while moving on the belt and in this case they may also be scanned while in free flight to obtain a determination of their positions and/or size as additional information which can be used to control the deflection apparatus. In both types of system, the scanning operation is commonly performed by transmission scanners having components located both above and below the rock stream. When dealing with ores contaminated with fine material it is found that the fine material is sprayed from the end of the conveyor belt and falls over the scanning equipment below the main rock stream. This can cause spurious scanning signals and physical damage to the scanning equipment. The present invention provides a sorting apparatus in which objects to be sorted are projected in a downward stream so that the fine material does not tend to separate from the main stream.
Quite apart from the problems caused by stray fine material, it is possible to stabilize ore rocks in a steady spaced apart relationship in a downward stream by means of equipment which can be simpler and considerably cheaper than the conventional equipment for depositing and stabilizing rocks on a fast moving horizontal belt. The invention may also find application in cases where the rocks simply pass an apropriate detector and are not optically scanned.